At least four people dead after vessel being escorted by Australian navy ships to Christmas Island rolled over.

A boat carrying about 150 asylum seekers has capsized in the Indian Ocean, spilling men, women and infants into rough waters and killing four people, Australian officials have said.

The boat rolled over on Tuesday in choppy seas 125km north of Christmas Island, Home Affairs Minister Jason Clare said.

The Australian island lies 500km south of Jakarta, Indonesia.

Two navy ships had been sent to assist the vessel earlier in the day, but rescue crews were unable to board as waves reached as high as four metres, said David Johnston, Commander of Australia's Border Protection Command.

The navy ships instead began escorting the boat to Christmas Island, where Australia operates a detention camp for asylum seekers.

Several hours into the journey, the boat started leaning and some passengers began jumping into the water.

Shortly after, the boat rolled and capsized, Clare said. Officials do not know what caused the boat to flip.

Rescuers dropped life rafts into the water and plucked 144 people to safety. The bodies of two women and two men were recovered before the search was called off on Tuesday night.

The survivors, from Afghanistan, Pakistan, Iran and Iraq, were taken to Christmas Island, along with three Indonesian crew members. There were several infants and children on board, Johnston said.

Customs officials estimated the boat had 150 passengers and said they are interviewing survivors to determine the exact occupancy of the boat but are no longer searching for more people.

Christmas Island is a popular destination for asylum seekers who crowd into rickety boats at Indonesian ports and pay people-smugglers to ferry them to Australian shores. Hundreds have died while attempting the journey in recent years.

Last week, another boat carrying asylum seekers sank off Christmas Island. Officials recovered the body of a baby boy and were unable to find eight passengers who vanished in the water.

Canberra is faced with record numbers of asylum seekers arriving on its shores, with the highly divisive issue dominating discourse ahead of September elections.

No extradition for 'people-smuggler'

Meanwhile, a local court in Indonesia has rejected Australia's request for the extradition of an alleged Afghan people-smuggler.

A Jakarta court on Thursday found "no legal foundation" to extradite Sayeed Abbas, ordering his release citing Indonesia's 1979 extradition law that did not cover the crime of people-smuggling.

Abbas' release was seen as a major blow to Australia as it seeks to reduce the number of asylum seekers arriving on its territory by sea and dismantle people-smuggling networks that organise the boat journeys.

"Sayeed Abbas will remain in detention up until the president makes his decision," prosecutor Mahayu Suryandari told AFP news agency, adding that the court ruling was also under review.

Australia has sought Abbas' extradition since 2010, accusing him of organising dozens of boats for asylum seekers, including one in 2011 that sank and killed some 200 people.

Australian authorities have said they believe Abbas continued to arrange boats for asylum seekers from his prison cell.

A court in Perth issued a warrant for his arrest on 27 charges related to people-smuggling to face a maximum jail term of 20 years.

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